Soil Compaction Impact and Modelling. a Review

Agron. Sustain. Dev. (2013) 33:291–309 DOI 10.1007/s13593-011-0071-8

REVIEW ARTICLE

Soil compaction impact and modelling. A review

Muhammad Farrakh Nawaz & Guilhem Bourrié & Fabienne Trolard

Accepted: 1 December 2011 /Published online: 31 January 2012 # The Author(s) 2012. This article is published with open access at Springerlink.com

Abstract Compaction of agricultural soils is a concern for consider existence of preferential paths of stress propagation many agricultural soil scientists and farmers since soil com- and localization of deformation in compacted soils. (7) Recent paction, due to heavy field traffic, has resulted in yield advances in physics of granular media and soil mechanics reduction of most agronomic crops throughout the world. relevant to soil compaction should be used to progress in Soil compaction is a physical form of soil degradation that modelling soil compaction. alters soil structure, limits water and air infiltration, and reduces root penetration in the soil. Consequences of soil Keywords Soil compaction . Soil disturbance . Soil stress . compaction are still underestimated. A complete under- Modelling . Soil degradation standing of processes involved in soil compaction is necessary to meet the future global challenge of food security. We review Contents here the advances in understanding, quantification, and 1. Introduction ...... 1 prediction of the effects of soil compaction. We found the 2. Causes of the soil compaction ...... 4 following major points: (1) When a soil is exposed to a 3. Quantifying the effects of the soil compaction ...... 5 vehicular traffic load, soil water contents, soil texture and 4. Effects of compaction on soil chemical properties and structure, and soil organic matter are the three main factors biogeochemical cycles ...... 7 which determine the degree of compactness in that soil. (2) 5. Effects of the soil compaction on plants ...... 9 Soil compaction has direct effects on soil physical properties 6. Effects of the soil compaction on soil biodiversity. .10 such as bulk density, strength, and porosity; therefore, these 7. Modelling ...... 11 parameters can be used to quantify the soil compactness. (3) 8. Remedies to the soil compaction ...... 14 Modified soil physical properties due to soil compaction can 9. Conclusion ...... 15 alter elements mobility and change nitrogen and carbon cycles in favour of more emissions of greenhouse gases under wet conditions. (4) Severe soil compaction induces root deforma- 1 Introduction tion, stunted shoot growth, late germination, low germination rate, and high mortality rate. (5) Soil compaction decreases Performance of soil on a particular land plays a vital role in soil biodiversity by decreasing microbial biomass, enzymatic the development and survival of civilizations as soil ensures activity, soil fauna, and ground flora. (6) Boussinesq equations the provision of food and further essential goods for humans and finite element method models, that predict the effects of (Hillel 2009). But the soil is a nonrenewable resource with the soil compaction, are restricted to elastic domain and do not potentially rapid degradation rates and extremely slow formation and regeneration processes (Van-Camp et al. 2004). So, the sustainable use of soils is the only solution to deal with the M. F. Nawaz : G. Bourrié : F. Trolard INRA, UR1119, Géochimie des sols et des eaux, global issues like food security, demands of energy and water, Aix-en-Provence, France climate change, and biodiversity (Lal 2009; Jones et al. 2009). Soil degradation is as old as agriculture itself; its impact * M. F. Nawaz ( ) on human food production and the environment is becoming Department of Forestry, University of Agriculture Faisalabad, Faisalabad, Pakistan more serious than ever before because of its extent and e-mail: [email protected] intensity (Durán Zuazo and Rodríguez Pleguezuelo 2008). 292 M.F. Nawaz et al.

Effects of soil degradation are not only on the livelihoods of observed at certain depth (subsoil compaction). Except a rural dwellers but it also poses a potential threat to global food few cases where a slight degree of top soil compaction can supplies over the long term (Scherr and Yadav 1996). Land be beneficial for some type of soils especially sandy soils degradation will remain an important global issue for the (Bouwman and Arts 2000), in most cases, it has negative twenty first century because of its adverse impact on agronom- effects on the soil. The subsoil compaction is a serious ic productivity, the environment, and its effect on food security problem because it is expensive and difficult to alleviate and the quality of life (Eswaran et al. 2001). The soil compac- and it has been acknowledged as a serious form of the soil tion is the physical form of soil degradation that changes the degradation by the European Union (Jones et al. 2003). soil structure and influences the soil productivity (Mueller et About 38% reduction in grain yield of wheat crop is al. 2010). Unlike salinity, water logging or the soil erosion that reported when the subsoil compaction was carried out at can be remarked from the soil surface, the soil compaction 0.15 m depth to a bulk density of 1.93 Mg/m3 (Ishaq et al. causes a hidden degradation of the soil structure that is diffi- 2001). The soil compaction in forests, due to mechanized cult to locate and rationalize (McGarry and Sharp 2003). operations, can be severe but shows more spatial variability Increased demands for the food and shelter have resulted in than in agricultural lands due to less systemic mechanized mechanization of forests and farms in almost all the developed operations and presence of stumps and heavy roots in the soil. countries as well as in many developing countries. Mecha- A number of reviews already exist on the soil compaction, nized operations involved in intensive cropping and in forest but they have been written many years back and are focused silvi-culture can, directly or indirectly, lead to the soil com- on specific aspects such as physical aspects of the soil compaction as shown in Fig. 1 (Ishaq et al. 2001; Silva et al. 2008). paction (Horn et al. 1995; Soane et al. 1982), the influence of About 68 million ha of the soils worldwide are estimated to be organic matter on the soil compaction (Soane 1990), model- affected by the soil compaction from the vehicular traffic. The ling the soil compaction (Lipiec and Hatano 2003;O’Sullivan soil compaction is responsible for the soil degradation in and Simota 1995), and the soil compaction by grazing animals Europe (33 million ha), Africa (18 million ha), Asia (10 (Drewry 2006). million ha), Australia (4 million ha), and some areas of North Some reviewed articles have also discussed the soil com- America (Flowers and Lal 1998; Hamza and Anderson 2003). paction on the basis of a specific land use, mainly crop systems The soil compaction can be defined as “the process by (Hamza and Anderson 2005; Soane and Van Ouwerkerk 1995) which the soil grains are rearranged to decrease void space and rarely forest systems (Greacen and Sands 1980). The most and bring them into closer contact with one another, thereby, recent review by Batey (2009) focused only on practical soil increasing the bulk density” (SSSA 1996). So, the soil management issues. In addition to the previous aspects by compaction involves the changes in physical properties of including the recent studies, this review also considers the the soil (bulk density and soil porosity) and these modified effects of the soil compaction on biogeochemical processes physical parameters of the soil are determinants of the and biodiversity, both at macro- and microscales. Furthermore, influence of the soil compaction on chemical properties of existing models for the soil compaction are critically discussed the soil, soil fauna, and diversity and plant growth (Fig. 2). and new directions for modelling the effects of the soil com- The soil compaction in cultivated lands affects mostly the paction on the soil are being proposed upper layer of soil (top soil compaction) but it is also 1.1 Description of the phenomenon

The soil compaction involves a microscopic rearrangement and bringing of the solid particles closer to one another and consequently an increase in the bulk density of the soil (Panayiotopoulos et al. 1994). But the degree of compactness is a quantitative parameter and defined as “the ratio of the actual bulk density to the reference bulk density obtained by uniaxial compression of wet soil (sufficiently for drainage) at static pressure of 200 kPa” (Håkansson 1990; Lipiec and Hatano 2003). The soil compaction is accompanied by the removal of the soil air, changes in the soil structure, and macroscopic increase in the soil strength (Taylor 1971). The phenomenon of the soil compaction can be explained in the classical elasto-plastic conception of stress–strain phenomena Fig. 1 Ruts formation after the passage of vehicular traffic on soil; an by considering the soil as a material that reacts elastically up to example of compacted soil a certain limit of stress; beyond that limit, any incremental Soil compaction impact and modelling. A review 293

Fig. 2 Causes of soil compaction and their effects on soil physical properties with ultimate direct effects on soil chemistry, plant growth and soil biodiversity while indirect effects on exchanges of matter with external compartments

stress results in the plastic deformation (Horn 1988). This bulk density of the soil (D) was established. When a loam- stress threshold for a given soil, under given climatic condi- Typic Haplaquept soil was subjected to varied pressures and tions, depends on soil texture, degree of aggregation, and moisture contents, it behaved totally differently from a loamy matric potential (Horn et al. 1995). The soil compaction, sand–Aquic Ustipsamment soil (Fig. 3). The former one was depending on the soil structure, influences soil physical, resistant to the compaction when dried and susceptible to chemical, and biological processes (Gupta et al. 1989; Fig. 2). compaction when moist to wet while the latter showed only Susceptibility of the soils to compaction varies with the small increases in compaction at incremental load and the soil texture. For example, the silt loam soils with low colloid moisture contents. Different behaviour in both types of soil contents are more susceptible than medium or fine textured is attributed to higher bulk densities of loamy sand soils when loamy and clayey soils at low water contents while the they are very dry due to the particles rearrangement with sandy soils are slightly susceptible to the soil compaction changing water contents (Smith et al. 1997). (Horn et al. 1995). In an experiment, Smith et al. (1997) Increases in the soil organic matter may reduce compatibil- selected 35 types of soils from timber growing areas covering ity by increasing resistance to deformation and/or by increasing a wide range of the soil textures (clay contents from 8% to elasticity (rebound effects; Soane 1990). High organic carbon 66%) and organic carbon contents (from 0.26% to 5.77%). A contents can even reduce the compactibility of soil at high vertical stress was applied on the soils by applying pressure of moisture levels in clay and silty clay soils (Smith et al. 1997). 0, 100, 200, 400, 600, 1,000, and 1,400 kPa at different water The soil compaction process is highly influenced by the contents and then bulk density was measured. Thus, a rela- soil water content (Hamza and Anderson 2005; Horn et al. tionship among pressure applied (P), water content (W), and 1995; Mosaddeghi et al. 2000). It affects the penetration 294 M.F. Nawaz et al.

Fig. 4 Relationship between soil water content and bulk density for maximum soil compaction. The error bars represent the standard deviation of mean values. From Ishaq et al. (2001)

pressure was applied on 35 soils of different textures (Smith et al. 1997). Knowledge of water contents in relation to the soil compaction for a particular soil can be helpful in scheduling the routine mechanical operations on that soil (Batey 2009; Ohu et al. 1989). The soil compaction can also be influenced by the state of energy of water, i.e., water potential, either matric or osmotic potential (Charpentier and Bourrié 1997). In nonsaturated conditions, the suction can influence compaction and the effect of the suction must be separated from the effect of the applied pressure (Cui et al. 2010). So, the soil water contents, soil texture and structure, and soil organic matter are the three main factors among others which determine the degree of compactness after the soil Fig. 3 Contradictory behaviour of two soils with different textures at is being exposed to vehicular traffic load. varied applied pressure and moisture contents. a Loam–Typic Haplaquept, b loamy sand-Aquic Ustipsamment. From Smith et al. (1997) resistance and load support capacity or maximum permissi- 2 Causes of the soil compaction ble ground pressure on the soil (Medvedev and Cybulko 1995). Vulnerability of a soil to compaction at the given soil Compaction can be a natural phenomenon (Fabiola et al. moisture and energy level depends also on its clay content 2003) caused by freezing and drying or an artificial phenom- and mineralogical characteristics (Smith et al. 1997; Wakindiki enon caused by the mechanical operations (Greene and Stuart and Ben-Hur 2002). Generally, a soil with very low moisture 1985). Conventional agricultural practices can also degrade content is less vulnerable to compaction than a soil with high the soil by the soil compaction (Quiroga et al. 1999). moisture content (Gysi et al. 1999). But when the moisture In modern agriculture, most of the field operations from content is so high that all the soil pores are filled with water, the sowing to harvesting are done mechanically by using heavy soil becomes less compressible (Smith et al. 1997). Using the wheeled machines which can compact the soil at every pas- bulk density as the soil compaction indicator, Ishaq et al. sage (Williamson and Neilsen 2000). The soil compaction by (2001) showed as to vulnerability of the soil to compaction a machine, in general, depends on the soil strength and loading increases with increasing water contents up to a limit after of machine (Alakukku et al. 2003). The soil strength is influ- which it decreases with the increasing water contents enced by the organic matter, water content, soil structure, and (Fig. 4). They carried out a laboratory experiment on the sandy texture while the loading is expressed by axle load, number of clay loam soil and found that the soil was compacted to its tyres, tyre dimensions, tyre velocity, and soil tyre interaction maximum at a soil moisture content of 120 g/kg. Similar (Kirby et al. 1997; Sakai et al. 2008). Axle load should not be results were reported in another experiment when a vertical confused with axle pressure as axle load is weight of machine Soil compaction impact and modelling. A review 295

(kilogram) while pressure is the axle load per unit surface area results in the soil compaction, increased bulk densities, (kilopascal) and in the soil compaction; the term pressure is decreased soil porosity and decreased organic matter contents used to express the disturbance on a soil. Increasing the (Marion and Cole 1996; Sarah and Zhevelev 2007). pressure on the soil increases the chances of the soil compac- Military operations or military training exercises in the tion (Gysi et al. 1999). Increasing the frequency of passages of past have also resulted in severe soil compactions in some machines over a soil increases its dry bulk density and cone places (Silveira et al. 2010) and increased bulk density of index resulting in the top soil compaction and unsuitable the soils up to 2.12 Mg/m3 has been reported due to military physical soil conditions for seed emergence (Botta et al. operations (Webb 2002). 2006; Sakai et al. 2008). However, a major portion of the total Natural causes (tree roots, precipitation, seasonal cycles, soil compaction is caused by the first passage (Bakker and etc.) of the soil compaction are not as harmful as anthropogenic Davis 1995; Silva et al. 2008) or early passages (Sakai et al. causes: the soil compaction associated with natural causes is 2008) of the machine and 10 passes can affect the soil up to limited in top 5 cm of the soil and the soil compaction due to 50 cm depth (Hamza and Anderson 2005). the trampling and urban pressure on a site can compact the soil Animal trampling can cause the soil compaction and can up to 20 cm while mechanical operations can compact the soil degrade the soil structure (Silva et al. 2003). The soil com- up to 60 cm. No matter of which origin it is, the soil compac- paction caused by grazing animals through hoof action is tion influences the water dynamics (Schlotzhauer and Price likely to be more widespread within the paddocks as com- 1999), pesticide diffusion (Alletto et al. 2010; Van den Berg et pared to the soil compaction caused by mechanical imple- al. 1999), soil erosion (Kosmas et al. 1997), carbon and nitro- ments which is limited under the tracks (Drewry 2006; Sigua gencycle(DeNeveandHofman2000), plant growth (Lowery and Coleman 2009). Physical deterioration by grazing animals and Schuler 1991), and mechanical operations cost (Soane and depends on the trampling intensity, soil moisture, plant cover, Pidgeon 1975); as we shall discuss in the coming sections. land slope, and land use type. Animal caused the soil compaction could range from 5 to 20 cm and might affect the soil bulk density, hydraulic conductivity, macropore volume, and 3 Quantifying the effects of the soil compaction penetration resistance of the soil (Hamza and Anderson 2005; Sigua and Coleman 2009). Effects of the grazing animals on To characterize the soil compaction, physical parameters the soil physical properties (Drewry et al. 2008), and soil such as the bulk density and porosity, soil strength, water nitrogen and carbon have been discussed in detail in literature infiltration rate, and reduction of aeration have been used. (Bhandral et al. 2007; Piñeiro et al. 2010). Indeed, under natural conditions, due to steady-state aggre- In contrast to the cultivated lands, harvesting operations in gation processes, and biological processes, the soil contains forest cause more soil compaction because of: (1) the use of a large proportion of macropores. The soil compaction can heavy machinery for harvesting; (2) felling, pushing, pulling, result in the destruction of inter-aggregate pores, in the and lifting of logs; (3) during transport of logs that exert a reduction of soil hydraulic conductivity and air permeability combined pressure on the soil; (4) no tillage operations in forests (Horn et al. 1995). Macropores are relatively more affected to loosen the soil. In the forests, harvesting operation causes during the soil compaction than micropores. different types of the soil disturbances and probability of the soil compaction is directly related to harvesting system and harvest- 3.1 Bulk density and porosity ing density (Sowa and Kulak 2008). Mostly severe soil com- pactioniscausedwhenthinningandclearfellingoperationsare Bulk density (dry soil mass per unit volume) is the most carried out with machines and these operations can compact the frequently used parameter to characterize the soil compaction soil up to the depth of 60 cm leaving the effects for more than (Panayiotopoulos et al. 1994), but in swelling/shrinking soil, it 3 years (Greacen and Sands 1980). A simple logging operation is recommendable to determine the bulk density at the standard in the forests can damage 20–30% of the forest land up to the moisture contents (Håkansson and Lipiec 2000). Typical resis- depth of 30 cm (Herbauts et al. 1996). The use of light weight tance indicators, used nowadays, are highly precise for the soil multifunctioning machines can reduce the passages and ulti- density measurements up to the soil depth of 20 cm while for mately the degradation of the soil (Radford et al. 2000). deep stratum, the stress state transducers with six earth pressure In the urban areas, urban parks and recreational sites receive gauges that measure three dimensional stresses can be useful large number of visitors and with increasing urban population, (Eguchi and Muro 2007). The bulk density is difficult to visitors’ pressure on these sites is increasing day by day (Frick measure in gravelly soils (Webb 2002). For an accurate mea- et al. 2007). Trampling effects of the visitors on the soil and surement of the effects of the soil compaction on all types of the vegetation have been reported by many authors (Jim 1987; soil, the soil bulk density alone is not adequate but other soil Sarah and Zhevelev 2007) and these effects are long term in properties such as the soil strength, soil aeration, and soil some cases (Kissling et al. 2009). Increasing visitors’ pressure moisture should also be measured (Lipiec and Hatano 2003). 296 M.F. Nawaz et al.

In an experiment on a clayey oxisol, Silva et al. (2008) Horn and Rostek 2000;Taylor1971). The soil strength increases analyzed the effects of the intensity of traffic on the soil with increasing bulk density while it decreases with decreasing compaction. They removed the 7-year-old Eucalyptus stand soil moisture content. One should be careful when measuring manually with chainsaw and soil was compacted with forest penetration resistance because it varies between the seasons due tractor, weighing 11,900 kg and loaded with 12 m3 wood, by to different moisture contents (Bouwman and Arts 2000). driving along same track zero, two, four, and eight times. They The soil strength is measured by a penetrometer (Usowicz found that the first two passes of forwarder caused maximum and Lipiec 2009) and, furthermore, cone penetrometer is widely increase in the bulk density and maximum decrease in infiltra- employed (Yu and Mitchell 1998) to measure the soil strength in tion rate. In other experiments, 30% increase in bulk density terms of cone resistance (megapascals). The cone resistance also was observed after mechanical clearing of the forests (Weert serves as an indicator of the root penetration and root growth 1974) and 20% increase in the bulk density was found after tree capabilities (Materechera and Mloza-Banda 1997).Sinnettetal. length skidding in pine hardwood stands (Dickerson 1976). (2008) reported that a soil having a cone resistance larger than Decrease in the soil porosity has been widely reported in 3 MPa caused a major hindrance for the root penetration of four the cultivated crops and forests after mechanical operations tree species (Japanese larch, Italian alder, birch, and Corsican (Dickerson 1976; Silva et al. 2008). Herbauts et al. (1996) pine) in the sandy loam soils as shown in Fig. 5; nearly all roots showed that a logging operation, in the loamy and acidic (90.7%) were present in the soil with a cone resistance class less soils with an illuvial and frequently mottled argillic B horizon, than3MPa. has increased the bulk densities and decreased the total porosity of the soils up to 30 cm depth at two different sites, Terrest 3.3 Water infiltration rate and Tumuli (Table 1). It is reported that an increase in contact pressure of 100 kPa caused a decrease of 5.7% in the soil Soil water infiltration rate can also be used to monitor the soil porosity at 10–15 cm depth after 24 passes in the sandy humus compaction status because the soil compaction reduces the rich forest soil (Sakai et al. 2008). total porosity of the soil (Silva et al. 2008), and mainly the number of macropores, water infiltrates faster in uncompacted 3.2 Soil strength soil than in a massively compacted soil of the same type (Hamza and Anderson 2003). These are not directly related The soil strength (resistance to penetration) is also widely used to the changes in porosity but rather to the changes in both the for the soil compaction measurement (Bouwman and Arts 2000; number of macro-pores and in the connectivity between

Table 1 Bulk density and total −3 porosity of eluvial and illuvial Horizon Depth (cm) Bulk density (kg dm ) Total porosity (%) horizons in the beech stands Core method studied (undisturbed vs. rutted soils) Terrest n010 Undisturbed soil E 10–30 1.37±0.08 48.4±3.1

Bt 30–50 1.66±0.04 37.3±1.6 Rutted soil

Eg 10–30 1.54±0.10 41.8±3.6

Btg 30–50 1.58±0.08 40.2±3.2

Tumuli n030 Undisturbed soil E 10–30 1.31±0.10 50.6±3.8

Bt 30–50 1.54±0.06 42.1±3.8 Rutted soil

Eg 10–30 1.62±0.07 38.9±2.8

Btg 30–50 1.54±0.05 41.8±1.8 Mean values at two different sites (Terrest site, n010; Tumuli site, n030) are given with stan- Bulk density (core method) dard deviations. From Herbauts Unpaired t test Terrest Tumuli et al. (1996) E vs. Eg −4.363*** −13.607*** *P<0.05, **P<0.01, Bt vs. Btg 2.584* NS ***P<0.001 Soil compaction impact and modelling. A review 297

Fig. 5 Mean percentage of roots in each penetration resistance class using the penetrometer; 90.7% of roots are present in penetration resistance class less than 3 Mpa. From Sinnett et al. (2008)

macropores (see below). Such changes in tortuosity can influ- air permeability also influence the soil chemical properties. ence the soil electrical conductivity (Seladji et al. 2010). The soil compaction causes decrease in oxygen diffusion (Renault and Stengel 1994) and can lead to anoxic conditions in 3.4 Reduction of aeration compacted soils if consumption of oxygen is faster than diffusion (Schnurr-Putz et al. 2006).Atthesametime,duetothe Reduced soil aeration can be an indication of the soil compaction reduced water infiltration rate, the soil compaction can result in and soil aeration can be quantified by different parameters such as the surface water logging in the wheel ruts covered areas during the air filled porosity, oxygen diffusion rate (ODR), redox poten- the wet seasons that can influence all the pedological processes, tial, and air permeability (Cannell 1977). Air permeability varies especially iron geochemistry (Munch and Ottow 1983). largely according to the soil physical properties for the same level Surface water logging and absence of oxygen, in compacted of compaction while the measurement of ODR by electrode soils; result in the lowering of redox potentials of soil solution, needs a lot of care. Redox potential measurements can be a good formation of reduced forms of iron (Fe2+; Ponnamperuma tool to characterize the compacted soils as these measurements 1985), increased dissolution of iron hydroxides and increase can be carried out in situ for the long periods, but this method is in organically complexed iron forms. Presence of iron minerals only applicable to the very wet soils (close to or at saturation; such as lepidocrocite that indicates hydromorphy can be ob- Feder et al. 2005; Lipiec and Hatano 2003;Nawaz2010). served in compacted soils by naked eye due to orange colours Among different methods discussed, the soil bulk density and of mottles but detection of these iron minerals by X-ray dif- the soil strength are more commonly employed to quantify the fraction (XRD) is not evident (Herbauts et al. 1996). In one soil compaction but the use of other indicators like water infil- experiment, Herbauts et al. (1996) reported higher concentra- tration rate, ODR, redox potential, etc. in combination with them tions of easily reducible iron Fe2+ in the above 30 cm of the can largely increase our understandings and results precisions. soils after a logging operation in a forest land (Fig. 6). Ex- Now sensors have also been developed to detect the location and changeable Fe2+ was extracted from a freshly sampled soil with depth of the hard pans in the real time that are equipped with four a hydroxylamine/potassium chloride solution and determined horizontal operating penetrometers for on-the-go sensing and colorimetrically using orthophenanthroline. They directly cor- mapping of the location and intensity of hard pan (Loghavi and related the presence of 15–30% of free iron in the form of easily Khadem 2006). Sensor systems to measure the soil compaction reducible form Fe2+ with the water logging as the result of the have already been reviewed (Hemmat and Adamchuk 2008). soil compaction. Selective extraction techniques using citrate– bicarbonate and citrate–bicarbonate–dithionite showed that the soil compaction under forest resulted in an increase of readily 4 Effects of compaction on the soil chemical properties extractable Fe oxides after only 2 years, before mineralogical and biogeochemical cycles transformations were detectable by XRD (Nawaz 2010).

4.1 Reductive conditions 4.2 Carbon and nitrogen cycles

Modified soil physical properties due to the soil compaction The soil compaction affects concentration of carbon dioxide such as the reduced water infiltration rate and reduced soil (Conlin and Van den Driessche 2000) and mineralization of 298 M.F. Nawaz et al.

their experiment, the soil was compacted by increased tractor weight up to a bulk density of 1.40 and 1.39 Mg/m3 at 0–100 and 100–250 mm depths, respectively, as compared to normal bulk densities of 1.21 and 1.26 Mg/m3 at the same depth, respectively. It is clear from Fig. 7 that heavy compaction

treatment gave greater response in term of N2O emission to rainfall than the zero compaction treatment. Bessou et al.

(2010) tried to model the emission of N2O gas after the soil compaction, but their model was not capable of capturing the emission during the cropping cycle. The soil compaction reduces the available N (Tan et al. 2008) and efficiency of N use by the crops decreases (Douglas and Crawford 1991), which can increase the fertilizer require- ments. It is reported that the soil compaction which ultimately increases the water contents and denitrification processes in

the soil, likely reduces the emissions of NOx from the soil (Skiba et al. 1994) but increases the volatilization of ammonia,

Fig. 6 Profile distribution of easily reducible iron (KCl-NH2OH- as compared to uncompacted soils (Soane and VanOuwerkerk extractable Fe+2) in undisturbed (transparent rectangles) and rutted soil 1995). (dark black rectangles). From Herbauts et al. (1996) The soil compaction favours the anaerobic soil conditions which can result in the increase in methanogenic (methane the soil organic carbon and nitrogen in the soil (De Neve producer) bacteria while decrease in the methanotrophic and Hofman 2000). In a laboratory experiment, when silt (methane oxidising) bacteria (Yao et al. 1999). The soil com- loam (acid forest soil) was compacted artificially to a bulk paction will result in the higher production rate of CH4 than its density of 1.5 from 1.1 Mg/m3, a significant reduction in the oxidation or destruction rate and this destruction rate can be carbon mineralization and net nitrification rates was observed reduced up to 58% when well drained soils are compacted after 9 months (Tan and Chang 2007). The soil compaction, (Soane and Van Ouwerkerk 1995). directly, results in the lower efflux of CO2 from compacted soils (Silveira et al. 2010) but, indirectly, due to increase of 4.3 Environmental impacts of the soil compaction machinery use to plough the compacted soil, can lead to more consumption of the fuel and ultimately more emission of CO2 Local soil compaction can influence not only the soil but (Voorhees and Hendrick 1977). also the local environment (Soane and Van Ouwerkerk Denitrification increases with the soil compaction (Arah 1995). The emissions of greenhouse gases due to the soil and Smith 1989) that results in the increased emission of N2O compaction (N2O, CH4, and CO2), as discussed in Section to the atmosphere (Douglas and Crawford 1993). These emis- 4.2 can enhance the greenhouse effect. The soil compaction sions can be much larger in the cultivated fields if N fertilizer results in increased energy costs in the cultivated lands due is applied in wet conditions (Clayton et al. 1994). In fact, in to the increased fertilizer inputs and greater tillage require- the soils, N2O is produced by both the nitrification (aerobic ments. However, it can also be responsible for energy savings, soil conditions) and denitrification (anaerobic soil conditions) in some soils, due to increase in machine efficiency in rolling and sometimes the nitrification and denitrification can over compacted soils (O’Sullivan and Simota 1995). Anaerobic occur simultaneously in the same soil aggregate (Davidson conditions in the soil due to the soil compaction can result in et al. 1986). As the soil compaction results in the increase reduced decomposition of pesticide and ultimately increased of water contents, so, it can increase strongly denitrification leaching of pesticide in groundwaters and aquifers (Alletto et processes in the soil (Maag and Vinther 1996). Soane and Van al. 2010). Similarly, decreased hydraulic conductivities can Ouwerkerk (1995) have reported that the soil compaction can result in slow downward movement of water and, ultimately, cause an increase in the denitrification rate and emissions of more nitrate contents in ground waters. N2O about 400–500%. But, in compacted soils, there is a If the soil compaction is carried out in steep slopes, this possibility of decrease of N2O transport to atmosphere and can result in increased runoff and ultimately in increase soil decreased reduction of N2OtoN2 gas, a harmless gas, depend- erosion and sediment transport which could be a serious ing upon the residence time of N2O in the soil and soil problem for the landscape. Furthermore, increased runoff, conditions (Soane and Van Ouwerkerk 1995). in slurry applied fields, can result in the entrance of slurry in It is reported that emission of N2O after fertilization is surface waters and ultimate threat to the aquatic life as highly dependent on the rainfall (Ball et al. 1999; Fig. 7). In degradation of slurry can reduce the oxygen levels in surface Soil compaction impact and modelling. A review 299

Fig. 7 Temporal variability of N2O fluxes and 3-hourly rainfall in heavily compacted soil and uncompacted soil under winter barley, shortly after the main spring fertilisation of 110 kgN/ha/year, assessed using the automatic chambers. From Ball et al. (1999)

waters. However, in some soils (sandy soils), the soil com- the soil compaction on roots generally vary with interspecies paction increases the soil strength, erodibility, and conse- and for different cultivars of the same species, due to difference quently the soil erosion for the same amount of runoff is in root penetration ability depending on the root physiology reduced. So, modified soil physical properties due to the soil and morphology (Materechera et al. 1991;Tardieu1994). compaction can be beneficial or harmful for the environ- Generally, compaction results in a decrease in the root ment depending upon the existing environmental conditions length, root penetration, and rooting depth (Glinski and and physical properties of the soil before modification. Lipiec 1990; Kristoffersen and Riley 2005). It is reported that the compaction of calcareous loamy soils, having 5% organic matter, with a load of 14.5 Mg resulted in complete 5 Effect of the soil compaction on plants failure of the root penetration in the deeper soils (>20 cm; Bouwman and Arts 2000). The soil compaction can also Overall effect of the soil compaction on the plant yield is aggravate a root disease in some species of plants (Fritz et negative (Ishaq et al. 2001; Saqib et al. 2004a) but it can also al. 1995). Top soil compaction is a more limiting factor for result in no effect or yield increase as reviewed by Greacen and the root growth than the subsoil compaction (Botta et al. Sands (1980). The soil compaction results in the restricted root 2006). The effects of the soil compaction on the ion uptake growth, decreased accessibility of nutrients, and increased loss and root growth are more severe in saline soils than in of the soil nutrients by leaching, runoff, and gaseous losses to normal soils. Saqib et al. (2004b) found that the compaction atmosphere which can affect plant growth. Effects of the soil of a sandy clay loam soil to a bulk density of 1.65 from compaction on uptake and losses of nutrients have already 1.21 Mg/m3 reduced root length density of wheat plants been reviewed (Lipiec and Stepniewski 1995). If a soil is while the presence of salinity (15 dS/m) was more drastic already suffering from other types of degradation such as the than the soil compaction alone. In the same experiment, salinity, drastic effects of the soil compaction on the plant they observed greater reductions in K+ concentrations and the growth and crop yield are reported to be doubled (Saqib et K+/Na+ ratio in leaves due to interaction of salinity and al. 2004a). compaction. The roots of some cover crops have shown good penetra- 5.1 Roots tion ability and less adverse effects of the soil compaction. These crops can be used to alleviate the effects of the soil Roots play an important role in the nutrient uptake and plant compaction (Rosolem et al. 2002). Because of larger diame- growth (Marschner 1986). Root penetration ability is ad- ters of roots than soil pores, roots can also increase the bulk versely affected by the soil compaction due to increased soil density of the soil near the roots during the root penetration strength and decreased number of macropores (Gerard et al. (Dexter 1987) and this phenomenon can change the physical, 1982). Soil strength–root relation is well documented and biological, and chemical aspects of the soil near the roots reviewed in literature (Hamza and Anderson 2005; Kirby (Glinski and Lipiec 1990). Change in micro- and mesoporosity and Bengough 2002; Masle and Passioura 1987;Tayloretal. around roots can also be quantified by scanning electron 1966; Taylor and Ratliff 1969;Voorheesetal.1975). Effects of microscopy (Bruand et al. 1996). 300 M.F. Nawaz et al.

5.2 Shoots Table 2 Effects of the relative degree of soil compactness (RDC) on barley shoot yield (gdrymatter/pot) and on P uptake (mgP/pot) in the three soil groups Although rooting system of the plants is badly affected by the soil compaction, this does not always result in reduced shoot Loam Clay loam Silt growth because it depends on the availability of nutrients in −1 the soil. If a soil is so heavily compacted that it reduces the Shoot yield (g pot ) RDC75% 8.6 5.6 4.8 mobility of the ions in soil and severely restricts the root RDC90% 7.4 4.8 3.4 growth; it can limit the shoot growth. Ishaq et al. (2001)and p Value 0.03 0.001 0.006 −1 Silva et al. (2008) observed no effects of the soil compaction P uptake (mg pot ) RDC75% 28.9 11.9 8.9 on the plant height but reduction in the grain yield was RDC90% 24.2 10.2 7.0 reported by Ishaq et al.(2001). p Value 0.005 0.003 0.02 From Kristoffersen and Riley (2005) 5.3 Seedling emergence

Seedling emergences are adversely affected by the soil com- severe soil compaction can result in the root deformation, paction (Dürr and Aubertot 2000). The soil compaction is more stunted shoot growth, late germination, low germination detrimental to the seedling growth and survival as compared to rate, and high mortality rate. All these impacts of the soil established plants and trees. Increase in the bulk density of a compaction contribute largely in reducing the yield of most 3 dry soil from 1.3 to 1.8 Mg/m in a greenhouse experiment agronomic crops in compacted soils. resulted in the late emergence of oak seedlings and a mortality rate of 70% (Jordan et al. 2003). In the same experiment, they found that the soil compaction resulted in reduced height of the 6 Effect of the soil compaction on soil biodiversity young seedlings and reduced N recovery. Similar findings were reported by different authors in the pot experiments and Modified soil physical parameters determine the effect of the field experiments (Corns 1988; Moehring and Rawls 1970; soil compaction on physical and chemical properties of the Tworkorski et al. 1983). But the response of seedlings growth soils and ultimately on soil biota. The soil compaction can be to the soil compaction is also subjected to the soil types and favourable to soil biodiversity and vice versa depending upon plant species because sometimes moderate compaction of the nature of the soil, climate, and extent of the soil compac- sandy soils can be useful to the seedlings growth of woody tion. Beylich et al. (2010) reported the negative influence of the plant species (Alameda and Villar 2009). soil compaction on microbial biomass and C mineralization above an effective bulk density of 1.7 Mg/m3. 5.4 Nutrients uptake

Generally, the soil compaction reduces the uptake of nutrients 6.1 Bacterial population due to the damaged roots but it also increases the contact between the roots and soil particles which may lead to the Soil microbial biomass is adversely affected by the soil com- rapid exchange of ions between the soil matrix and roots. The paction (Frey et al. 2009; Pupin et al. 2009). The soil compac- – uptake of nutrients transported by diffusion is more affected tion resulted in reduced soil aeration of the soil due to 13 36% by compaction than for nutrients transported by mass flow decrease of air filled porosity which led to the reduction in (Arvidsson 1999). The soil compaction can decrease the up- microbial biomass carbon and microbial biomass nitrogen (Tan take of phosphorus and potassium in the maize (Dolan et al. and Chang 2007). Tan et al. (2008) also reported the reduction 1992) or can increase the uptake of phosphorus in the ryegrass of microbial biomass phosphorus after the soil compaction. and maize (Shierlaw and Alston 1984) depending on the type Shestak and Busse (2005) reported that the soil strength values – of the soil and nature of the soil compaction. Kristoffersen and ranging 75 3,800 kPa changed the physical properties of the Riley (2005) subjected three types of soils (loam, clay loam, soil but did not affect any biological indicator of the soil and silt) to relative degree of compactness (RDC) of 75% (microbial biomass and enzymatic activity). (RDC75%) and 90% (RDC90%) of the standard degree of compactness. They observed that heavy soil compaction 6.2 Enzymatic activity reduced the P uptake and yield of barley in all three types of the soils (Table 2). Any disturbance or stress to the soil can influence enzymatic So, the soil compaction negatively affects the root portion activities in the soil (Buck et al. 2000). The soil compaction of the plants but ultimate effect on the shoot depends on the changes physical and chemical properties of the soil which nutrient availability and uptake by the plants. However, leads to the reduction of phosphatase, urease, amidase, and Soil compaction impact and modelling. A review 301 dehydrogenase activities (Dick et al. 1988;Jordanetal. biomass, enzymatic activity, soil fauna, and ground flora in 2003;Pupinetal.2009; Tan et al. 2008), but some- compacted soils. times increase in the phosphatase activity is also reported (Buck et al. 2000). Anoxic conditions in the soil induce the changes in the microbial community and 7 Modelling favour organisms capable of tolerating these conditions, thus, lower eukaryotic/prokaryotic ratios, more iron and Modelling not only provides a better way to quantify the sulphate reducers, and higher methanogens were found in processes involved in the soil compaction but also helps us compacted soils than in uncompacted soils (Schnurr-Putz et to predict the vulnerability of a particular soil to compaction. It al. 2006). (modelling) is useful in the organisation and integration of existing knowledge and identification of gaps in knowledge. It 6.3 Larger soil fauna (modelling) is a simulation of all the processes involved in the soil compaction but soil compaction depends on a lot of Soil fauna plays an important role in the decomposition parameters and considering each parameter is difficult for and incorporation of organic matter in the soil (Petersen heterogeneous structures of the soil. Modelling of the effects and Luxton 1982). Habitat of the soil fauna is intersti- of the soil compaction on the environment and plant growth tial spaces in the soil. The soil compaction changes the are reviewed and discussed in detail in literature (Clausnitzer pore size availability and distribution which generally and Hopmans 1994;Grant1993;O’Sullivan and Simota leads to the reduction of the proportion of large pores 1995). Several attempts have been made to model the effects and affects the movements of nematodes and larger soil of mechanical operations on the soil (Blackwell and Soane fauna. Nematodes, being diverse in food habit (bacter- 1981; Défossez and Richard 2002;DicksonandRitchie1993; ivores, herbivores, and omnivores), play an important Raper and Erbach 1990), but most models have limited appli- role in the soil food web as well as in organic matter cations due to a large number of parameters as input or decomposition, nutrient decomposition and herbivory heterogeneous field conditions. Models can also be classified (Bouwman and Arts 2000). Heavy soil compaction and discussed as mechanistic or empirical, depending on may not affect the quantity of nematodes in the soil the treatment of underlying mechanisms, and determin- but can influence their distribution. Bouwman and Arts istic or stochastic, depending on the treatment of variability (2000) reported reduction of bacterivore and omnivore (O’Sullivan and Simota 1995). nematodes while increase of herbivore nematodes in heavily compacted soils. Earthworms are also reported – to be influenced by the soil compaction (Kretzschmar 1991; 7.1 Stress strain models based upon boussinesq equation Radford et al. 2001) and their population decreases with – increase in the soil compaction (Chan and Barchia 2007), Most of the models are based on the stress strain theory but they are capable to penetrate a soil with penetration where two problems are addressed: resistance of 3,000 kPa by ingesting the soil particles (Dexter & The propagation of stress in the soil 1978). & The local relation between stress and strain i.e.; the “constitutive equation” 6.4 Ground flora The propagation of stress in the soil is classically described Ground flora is very important in the forest ecosystem in by some form of the Boussinesq equation (Boussinesq 1885, terms of revegetation, productivity, aesthetics, and water p. 104), and a constant linear relation between stress and strain and nutrient cycling (Gilliam 2007). Any disturbance to is assumed, that is, soil reacts elastically. The form of the the forest ecosystem and/or soil affects adversely the native Boussinesq equation depends on the limiting condition. For ground flora (Zenner et al. 2006; Demir et al. 2008), but a point load (Fig. 8), it is: some plant species are capable to show healthy habitat and a σ ¼ = p 2 3θ ð Þ rapid recovery after extreme degradation of the soil (Demir Z 3P 2 r cos 1 et al. 2008). Zenner and Berger (2008) reported that the soil compaction resulted in shifting of ground flora from interior where, r is the radial distance from point A to the origin O forest species to noxious/invasive and disturbed forest spe- where the load P is applied, and θ is the angle between OA and cies and relative resistance of the initial ground flora to the vertical, σZ is the vertical stress. change was found to be linearly related to relative resistance In this equation, time is absent, and, therefore, it describes to penetration. The soil compaction influences the soil bio- the situation at mechanical equilibrium in static conditions. diversity negatively and it results in decrease in the microbial Moreover, as underlined by Smith et al. (2000), the stress 302 M.F. Nawaz et al.

A recent analytical model is SoilFlex, easily usable, is based upon a description of the upper boundary condition (load of tyre) as an ellipse or a super ellipse, considering both normal and shear stresses, an analytical solution to compute the stress propagation and a calculation of the soil deformation (Keller et al. 2007; Keller and Lamandé 2010). According to Keller et al. (2007), “A weak point of the analytical solution may be the concentration factor, as it is not a directly measurable soil parameter.” According to Smith et al. (2000), the concentration factor fitted “can result in inaccurate results if they are used for comparing strength among different soils [....], and it is a machinery-soil dependent parameter, [influenced by] inflation pressure, tires dimensions, lugs and carcass stiffness”. “ ’ Fig. 8 Stress propagation to a layer parallel to the surface at depth z for These latter authors concluded that Boussinesq sequa- a homogeneous isotropic soil, under the assumption of elasticity, in a tions, modified by concentration factors and elliptic soil submitted to a point load P exerting a normal stress at the surface. coordinates failed to predict experimental stress values From Défossez and Richard (2002) in a Hapludand.” In addition, the Boussinesq equation and its classical modifications are restricted to a boundary condition of normal “ distribution is irrespective of differences in texture, bulk den- stress while Boussinesq proposed other integrals applicable ” sity or water content . To better describe the stress distribution, to tangential forces which due to the linearity of the differential “ ” the concentration factor , v was introduced by Fröhlich (1934; operators can be combined to give general solutions for any quoted by Défossez and Richard 2002), so that the equation external stresses; the solutions are derived from potentials that becomes: are: (1) ordinary, ʃ (dm/r), when displacements are known at 2 v the boundary surface; (2) logarithmic ʃ ln(z+r)dm, when nor- σZ ¼ nP=2 p r cos θ ð2Þ mal stresses are known; (3) logarithmic ʃ [−r+zln(z+r)]dm, whichmeansthat,whencomparedtoEq.1, the geometric when stresses at the surface are purely tangential (Boussinesq coefficient 3 is treated as an adjustable parameter. When the 1885, p. 201). “concentration factor” increases, stress increases at a given The major problem with Boussinesq’s theory is that it is point. Söhne (1958; quoted by Défossez and Richard 2002) restricted to elastic domain which implies that there is no suggested n values of 4, 5, and 6 for hard, firm and soft soil permanent deformation and no rupture and the solid is respectively, and Défossez and Richard (2002) commented as: supposed to be homogeneous and isotropic. Moreover, it “The firmness results from empirical combinations of both the does not represent accurately hydraulic properties of the soil bulk density and water status of the soil.” According to Smith and cannot describe the soil deformation. Furthermore, these et al. (2000), the concentration factor can even obtain values of methods fail to predict changes at pore-scale level (Or and 6–9, and is influenced by the soil structure: “in well-aggregated Ghezzehei 2002). soils, the concentration factor values are smaller than in the same but homogenized soils.” They used even values smaller 7.2 Virtual work formulation than3(v01), in simulations, and calculated values from 1.5 to 2.8 for different decreasing laws of the tire load from just below A different way of computing the propagation of stress is the tire centre to its external limit. based upon a local description of the virtual work: Analytical models based upon Boussinesq equation and on Z Z I its modifications are largely used as they demand less number d"T σdV ¼ duT pdV þ duT tdA ð3Þ of inputs as compared to models based on the finite element method (FEM). The comparison with experiments largely where, V and A are the volume and the area of the surface of gives variable results, acceptable for homogeneous soils and the deformed body, σ and " the tensors of stress and strain, unreliable for heterogeneous soils due to the presence of clods du is the incremental displacement, p and t are respectively or a firm soil at depth (Défossez and Richard 2002). In the body forces and surface traction, and the superscript T homogeneous soils, they can predict efficiently not only soil stands for transformed (Défossez and Richard 2002). stress–strain behaviour but also the propagation of the loading Time is equally absent from the equation and it describes forces within the soil resulting from forces applied at the soil static deformation of a soil body. This equation is then surface from farm vehicles. linearized which assumes low deformation and numerically Soil compaction impact and modelling. A review 303 solved, using finite element methods. The corresponding There have been advances in physics of granular media, models are referred to as FEM. These models are adequate due to both fundamental interest for physicists as models of for modelling the 3D distribution of stress within the soil much more complex systems and to practical and/or industrial induced by wheeling and the complex stress–strain behaviour interest: “granular materials are ubiquitous in nature and are of the soil but due to continuous changes of elastic parameters the second most manipulated material in industry (the first one of the soil, application of FEM models becomes limited is water)” (Richard et al. 2005). One of the main character- (Raper and Erbach 1990). istics of granular materials is that their behaviour is interme- Whether the stress propagation is computed by a diate between solids and fluids. Compaction from a loose- pseudo-analytical procedure (Boussinesq and its var- packed material can be efficiently obtained by tapping and iants) or by FEM, it fails to account for two evidences: shearing, and this is more efficient than compression. Granu- the existence of preferential paths of stress propagation lar packings submitted to gentle mechanical taps can reach a and the localization of deformation (hard pans, plough stationary configuration which does not depend on the initial pans…). These items will be addressed in the following conditions (looser packing or denser packing; Ribière et al. paragraphs. 2007). As friction between solid particles oppose to mixing and thermal agitation is entirely negligible with respect to 7.3 Preferential paths of stress propagation potential energy due to gravitation (>1×1012 kT), solid particles can segregate which is well-known in soils, though at Stresses do not often propagate homogeneously but through first sight, it could be considered as violating the natural preferential paths, isolating bulk volumes that are not under so tendency for entropy to increase. Those materials are, thus, large stress as it is in the preferential path. This is due to the fact considered as “a-thermal” and metastable assemblages can that in soils, there coexist different assemblages due to small persist as long as no perturbation occurs (Jaeger et al. 1996). differences between the size and shape of the particles. When This metastability of different assemblages explains in a large submitted to a compression, the soil particles or grains will tend part soil heterogeneity. Forces in such materials at rest appear to move at first elastically. Soon, some of these grains will be to be very heterogeneous, forming chains along which stresses blocked (“jammed”) against each other and normal stresses are very intense (Majmudar and Behringer 2005). Those will be transmitted along chains of preferential propagation. chains isolate volumes which are not under stress forming When these chains constitute a continuous path, by a percola- arches, as is well-known in silos. This behaviour is strongly tion process (Bideau and Hansen 1993; Guyon and Troadec influenced by the shape and rugosity of particles. When 1994;Rouxetal.1993), they will isolate bulk volumes sub- compression proceeds further, deformation can be localized mitted to smaller stress or even free to move (Fig. 9). in specific locations.

Fig. 9 Preferential paths of stress propagation. From Majmudar and Behringer (2005): comparison of experimental images (a, c) and computed images (b, d).Top pair a low-force sheared state. Bottom pair a high-stress isotropically compressed state 304 M.F. Nawaz et al.

7.4 Strain localization are determined by an interpolation procedure between the responses for axisymmetric triaxial states. The for- There exists a broad evidence that strain is not evenly distrib- mation of a shear band is treated in model CLoE as a uted in soils, rocks, and geomaterials such as concrete. Indeed, bifurcation problem. The appropriate 16 parameters for strain localization is rather a rule. It is generally associated a given material are derived from simple axisymmetric with plastic deformation and ruptures in solids and is observed triaxial compression and extension tests except out-of- to concentrate in narrow zones, called shear bands (Desrues axes shear moduli which are derived from special ex- and Chambon 2002). Such localized deformations have been perimental tests combined with inverse analysis (Desrues and observed in many granular materials, from sand (Desrues and Chambon 2002). Viggiani 2004) to clays, e.g., by X-ray tomography (Bésuelle Time is, thus, present in this model and this accounts for et al. 2007;Fig.10). the fact that the rate of stress is a parameter of a paramount The material undergoes a transition from a diffused strain importance. The discrepancies between laboratory tests and mode to a localized strain mode where strain is strongly field wheeling experiments have been ascribed to the differ- spatially concentrated while the material outside this zone ences in loading time (Keller and Lamandé 2010). In the behaves approximately as rigid (Bésuelle et al. 2007). In soils, soils, the localization of strain in shear bands separated by this is the case for example in hard pans. And, at a much larger volumes behaving as more rigid can explain the observation scale, this is the basic paradigm of plate tectonics. This feature of discrepancies when modelling the soils with clods or with seems, thus, very general. Looking back at Boussinesq Eq. 1, an underlying dense layer at depths less than 0.5 m (Défossez equation structure precludes the existence of a maximum in and Richard 2002). stress and strain at a specific location as second derivative of Boussinesq equations and FEM models are restricted to this equation is always positive. It is interesting to note that the elastic domain and fail to take account of existence of prefer- presence of an inclusion, whether weaker or stronger than the ential paths of stress propagation and localization of deforma- bulk material, dictates the location of the shear band (Desrues tion in compacted soils. Modified forms of a constitutive and Viggiani 2004). Chambon et al. (1994, 2000) proposed a model like CLoE that is based upon a stress rate/strain rate constitutive model based upon a stress rate/strain rate relation- relationship in granular media can be able to decrease discrep- ship instead of a stress/strain relationship. It is a continuous ancies in modelling the soil compaction when there is localised model like the aforementioned models which means that the soil deformation. distances considered are much larger than the grain size and is called CLoE, formed on the words consistency and explicit localization. Failure is accounted for by incorporating explic- 8 Remedies to the soil compaction itly a limit surface in stress space separating admissible states from inaccessible states. The constitutive equation is: Natural phenomena involved in the recovery of compacted : soils are precipitations, wetting and drying cycles, subse- : : : – σ ¼ A : " þbkk" ð4Þ quent soil cracking, freeze thaw cycles, and bioturbation : which includes earthworm burrowing and root penetration where, σ is the stress rate, " the strain rate, A is a fourth-rank and decay (Drewry 2006; Webb 2002). Natural recovery of tensor and “b” a second-order tensor; the incremental : : compacted soil is a very complex and slow process that can no-linearity is due to the norm kk" ; " is not decomposed into take at average from 5 to 18 years depending on the soil type, elastic and plastic parts. A and b depend on state variables and degree of compaction, and climate (Froehlich et al. 1985).

Fig. 10 Strain localization in shear bands. From Bésuelle et al. (2007). Left initialization of the shear band. Right development of the shear band Soil compaction impact and modelling. A review 305

Among all the aforementioned factors, the degree of compac- of granular materials are promising but until now restricted to tion or bulk density is the most important factor to monitor the very simple systems. A link between those two approaches is, recovery time of a soil (Heinonen 1977). If the soils are not in principle, possible and is indeed the need of the time. Many highly compacted, repetitions of alternative dry and wet periods recent reviews suggest that the future research should focus can reduce the soil compaction in the clay soils but the sandy more on dynamics of loading and the data acquired could be compacted soils are less affected by these natural restoration treated with dynamic models, such as CLoE, relating stress rate cycles. Rapid natural amelioration of physically deteriorated and strain rate. topsoil to about 5 cm is possible but below 15 cm natural The soil compaction is rapid and easy due to mechanisation, rejuvenated process is very slow (Drewry 2006). For example, but it takes years to restore a compacted soil. In spite of full recovery time for a heavy compacted soil can range from hundreds of articles appearing during the last 10 years on the 100 to 190 years (Webb 2002). soil compaction, there is an urgent need to apply multidisci- Compaction can be reduced by the natural methods plinary approach in the soil compaction studies, addressing through increase of vegetation and addition of organic matter diverse effects in different soil compartments (Fig. 2). Progress by preventive measures through controlling traffic and animal in sensors in both the soil physics and soil chemistry and in data load or by mechanical methods by deep ripping (Berg 1975) treatment should be of a great help to evaluate the effects of the and disking (Dickerson 1976). Aforementioned solutions soil compaction on every compartment of the biogeosphere. have been reviewed in farm systems (Hamza and Anderson 2005). In the forests, any mechanical work to reduce the soil Acknowledgments The support of the Higher Education Commission compaction is difficult due to presence of stumps and large (HEC) of Pakistan and the Société Française d’Exportation des Ressour- roots, so, natural methods are encouraged and employed. ces Éducatives (SFERE) for the grant for MF Nawaz are gratefully acknowledged. Pr. D. Bideau and Dr P. Défossez are greatly thanked for fruitful discussions.

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